Method of silica optical fiber preform production

ABSTRACT

A method is disclosed for the manufacture of optical fiber preforms using plasma enhanced chemical vapor deposition (PECVD). The invention consists of a cylindrical reactor in which material such as flourine-doped silica glass is deposited on a cylindrical silica rod. A furnace for regulating reactor temperature encases the reactor. A microwave generator coupled with a resonator and an H 10  waveguide delivers microwave energy to the reactor, producing simultaneously symmetrical excitations in the E 010  mode and a plasma surface wave in E 01  mode located at the surface of the rod. A microwave plasma is scanned along the length of the rod through a slit in the reactor to deposit a homogeneous film of a desired thickness. The benefits of the present invention over the prior art include increased absorption of delivered power, and the ability to uniformly deposit films such as flourine-doped silica on rods with diameters of up to 30–35 mm and thus produce optical fiber preforms with diameters greater than 40 mm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to methods of silica preform manufacturing, inparticular for creation of optical fiber lightguides with reflectivecladding deposited by microwave plasma enhanced chemical vapordeposition (PECVD).

2. Information Disclosure Statement

Optical fibers are currently manufactured through a drawing process,where fibers are drawn from a preform with a large diameter. Thesepreforms are generally high purity glass or plastic. The fiber claddingis often applied to the preform prior to drawing the fiber.

The most common methods for the manufacture of fiber preforms involvechemical vapor deposition (CVD), which entail the use of vaporized rawmaterials that combine with oxygen and solidify into glass. Theprinciple CVD methods can be grouped into two categories. The first isthose methods that utilize thermal energy to create the precursor vapor,and includes modified chemical vapor deposition (MCVD), outside vapordeposition (OVD), and vapor axial deposition (VAD). The second utilizeselectromagnetic radiation to ionize precursor gas, thus forming a plasmafrom which the glass is deposited. Method in this category includeplasma CVD (PCVD) and plasma enhanced CVD (PECVD).

U.S. Pat. No. 6,138,478 by Neuberger et al discloses a method and devicefor silica preform production by microwave plasma deposition of anSiO₂—F cladding on a silica rod. The invention uses microwaves with afrequency of 2,450 MHz. This method is limited in that it cannot producea uniform deposition on a silica rod with a diameter greater than 25.This limitation is due to the nonsymmetry of E₀₂₀ mode excitation andarising nonsymmetrical wave of TE type. Microwave power losses onirradiation of large diameter silica rods and a hole in the reactor canbe up to 20% of the incident microwave power. This leads todeterioration of preform quality for large rods, and thus to arestriction on preform diameter. It is also impossible to increaseproductivity, as determined by deposition rate and silica rod diameter,by this method.

The closest analog to the present invention is disclosed in U.S. Pat.No. 5,597,624 by Blinov et al. A method of PECVD is described wherein asurface plasma wave of either the symmetric E₀₁ or the hybrid HE₁₁ typeis excited on the outside surface of a dielectric starting body, such asa silica tube. However, this method cannot be used in commercial-scalemanufacturing of large diameter and high quality silica preforms becauseof a lack of high power impulse microwave sources (both generators andamplifiers) that deliver microwaves in the 2450 MHz region with a 10 kWaverage power and 1 ms impulse duration.

The present invention is also useful for preventing hydrogen diffusion,or corrosion of the cladding due to environmental conditions, which canbe especially severe in high temperature applications. Adverseenvironmental conditions combined with stress serve to exacerbate thisproblem. High optical losses due to hydrogen diffusion are found inknown silica optical fibers. To prevent hydrogen diffusion, and thusprotect the fiber and extend its useful life, a buffer SiO_(x)N_(y)layer is typically applied. Generally, the thickness of such a layer isin the range of 100–10,000 A depending on the optical fiber application.Other SiO_(x)N_(y) layer thicknesses, up to a few microns, can beproduced if needed.

Although deposition of an SiO_(x)N_(y) layer is known and used toprevent hydrogen diffusion, modern sputtering or deposition equipment isexpensive and these devices and methods fail to generate a homogeneouslayer. Additionally, in present methods the deposition process issynchronized with the drawing of optical fibers from a silica preform.Applying an SiO_(x)N_(y) layer during drawing necessitates a decrease inthe fiber drawing rate, and further results in a decrease in processproductivity and an increase in the basic cost of fibers. This leads toconsiderable reduction of production efficiency (especially for preformdiameters within 30–40 mm).

A method of depositing SiO_(x)N_(y) layers during the manufacture ofoptical fiber preforms, so as to prevent hydrogen diffusion, is known,and is described in Japanese Patent No. 62-65948 by Akira et al. Thismethod eliminates the need to deposit SiO_(x)N_(y) layers during fiberdrawing and thus eliminates the production efficiency problems describedabove.

However, this method requires a two-stage process and has a lowefficiency in the deposition of chemical reagents (less than 50% ofgaseous reagants are actually deposited). Two setups are required foruse in this process. The first is a device for the deposition of a sootSiO₂ layer by MCVD, VAD, OVD on a preform surface. The second is adevice for vitrifying the soot in an atmosphere of N₂ and He. Thisprocess is rather long, and Helium is expensive. The basic drawback ofthis method is the application of high temperature depositiontechnologies (MCVD, VAD, OVD) that do not produce effective N₂dissociation (even at temperatures exceeding 2000° C.) in a gas phase oreffective N₂-doping of synthesized SiO₂ glass layers.

OBJECTIVES AND BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a plasma CVD fiberoptic preform manufacturing method that is capable of producing preformsof a larger diameter than known methods and devices.

It is another object of the present invention to provide a plasma CVDfiber optic preform manufacturing method that is capable of producingpreforms of a higher quality than known methods and devices.

It is still another object of the present invention to provide a plasmaCVD fiber optic preform manufacturing method that has higher productioncapabilities than known methods and devices.

It is a further object of the present invention to provide a plasma CVDfiber optic preform manufacturing method that increases the amount ofabsorbed microwave power per unit plasma volume (W/cm³) at the permanentfrequency of a microwave generator, thereby depositing a preformcladding layer in a shorter time with a more efficient use of microwavepower and precursor gas.

Briefly stated, the present invention discloses a method for themanufacture of optical fiber preforms using plasma enhanced chemicalvapor deposition (PECVD). The invention consists of a cylindricalreactor in which material such as flourine-doped silica glass isdeposited on a starting body such as a cylindrical silica rod. A furnacefor regulating reactor temperature encases the reactor. A microwavegenerator coupled with a resonator and an H₁₀ waveguide deliversmicrowave energy to the reactor, producing simultaneously symmetricalexcitations in the E₀₁₀ mode and a plasma surface wave in E₀₁ modelocated at the surface of the rod. A microwave plasma is scanned alongthe length of the rod through a slit in the reactor to deposit ahomogeneous film of a desired thickness. The benefits of the presentinvention over the prior art include increased absorption of deliveredpower, and the ability to uniformly deposit films such as flourine-dopedsilica on rods with diameters of up to 30–35 mm and thus produce opticalfiber preforms with diameters greater than 40 mm.

The above, and other objects, features and advantages of the presentinvention will become apparent from the following description read inconjunction with the accompanying drawing.

BRIEF DESCRIPTION OF FIGURES

FIG. 1—Illustration of the present invention through a plane parallel tothe central cylindrical axis.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present method consists of creating and moving a microwave plasma,in a low pressure reactor, along a silica rod with a high diameter(30–35 mm) by means of simultaneous symmetrical excitations of E₀₁₀ modewaves and plasma surface waves (PSW) of E₀₁ type. The E₀₁ type plasmasurface wave (PSW) is located on the interface between the silica rodand the plasma, where the electric field strength is maximum. Theelectric field strength exponentially decreases as it extends radiallyboth to the wall of the reactor and to the center of the rod. Thus, thereactor (silica tube) wall is located in a region where the electricfield is at a minimum. The E₀₁₀ mode in the cavity and the E₀₁ type PSWalong the surface of the silica rod are created simultaneously in onedevice by symmetrical and homogeneous excitation from a single microwavepower source through a special system of coupled inputs of energy.

In this case, microwave power losses on irradiation through the silicarod and holes in the resonator are reduced and specific absorbedmicrowave discharge power increases because more energy is absorbed bythe plasma due to E field intensity distribution and is used in reagentexcitation rather than being lost through absorption by the rod, thewall, or holes in the resonator. The present invention deposits acladding material, such as SiO₂—F, of uniform thickness on a silica rodand features a higher deposition rate than is available with the priorart. This device is capable of producing preforms with diameters up toand greater than 40 mm. Because of its high efficiency and its abilityto produce large diameter preforms, the present invention cansignificantly reduce the manufacturing cost of two-layer silicalightguides with cladding.

A preferred embodiment of the present invention is more particularlydescribed in conjunction with FIG. 1. A device used to accomplish E₀₁₀mode and E₀₁ type PSW excitation followed by preform depositioncomprises the following basic components: Silica rod 101, reactor 102,resonator 103, coupled opening 104, H₁₀-waveguide 105, and deliveryH₁₀-waveguide 106.

Cylindrical silica rod 101, whose diameter is up to 30–35 mm, is locatedwithin reactor 102. Reactor 102 is a hollow rod, preferably silica, witha diameter of 50 mm in a preferred embodiment. Both rod 101 and reactor102 are mounted coaxially with and inside a vertical cylindrical furnacefor homogeneous heating. Typical furnace temperatures are preferably inthe approximate range of 1100–1200° C. A microwave generator couplesmicrowave radiation to waveguide 105 for delivery via delivery waveguide106. Waveguide 105 in turn delivers radiation to resonator 103, where anE₀₁₀ mode is excited through a system of coupled openings in resonator103. The waveguide-generator-resonator apparatus can be scanned forwardand back along silica rod 101 along the cylindrical axis of rod 101through a slit in the furnace. The radial electric field intensitydistribution resulting from the combination of wave mode E₀₁₀ and plasmasurface wave E₀₁ exponentially decreases as the field extends radiallyfrom the surface of rod 101 towards reactor 102 and towards the centerof rod 101. Thus, silica rod 101 is located in the region of maximumelectric field value (E₀₁₀+PSW E₀₁), and reactor 102 is located in aregion of minimum electric field value.

In a preferred embodiment, deposition of flourine-doped silica glass iscarried out by thermal or microwave plasma-chemical treatment of silicarod 101 and reactor 102. As a preliminary step, microwave discharge 107is ignited by the addition of pure oxygen. A precursor gas mixtureconsisting of SiCl₄+O₂+C₃F₈ (or other suitable F-Compounds) is thendelivered into the cavity between rod 101 reactor 102. The operationpressure within reactor 102 during the deposition process is preferablymaintained at about 5–10 torr. The operation pressure is variable, andcan be modified as seen fit to accommodate a desired gas flow, microwavepower level and design deposition rate. The appropriate thickness of theresulting SiO₂—F cladding is dictated by the predetermined cladding/coreratio and predetermined fiber diameter. Generally, a preferredcladding-to-core thickness ratio is between about 1.06–1.4. Examples oftechnical process parameters that could be used in this preferredembodiment are presented below. The following examples are purelyillustrative, and do not limit the present invention to thoseparameters.

EXAMPLE 1

The silica rod diameter, mm 30 The reactor diameter, mm 50 The length ofdeposition zone, mm 1200 Frequency of generator, MHz 2400 Generatorpower, kW 5.6 Speed of movement of plasma generator, kW 2.0 Oxygen flow,cm³/min 6000 Freon Flow C₃F₈, cm³/min 150 SiCl₄ flow, cm³/min 1450 Gaspressure, torr 6.0 Reactor Temperature, C. 1150 Preform diameter, mm(ratio is 1.2) 36

EXAMPLE 2

The silica rod diameter, mm 35.0 The reactor diameter, mm 50.0 Thelength of deposition zone, mm 1200 Frequency of generator, MHz 2450Generator power, kW 6.0 Speed of movement of plasma generator, kW 2.0Oxygen flow, cm³/min 7500 Freon Flow C₃F₈, cm³/min 180 SiCl₄ flow,cm³/min 1800 Gas pressure, torr 8.0 Reactor Temperature, C. 1150 Preformdiameter, mm (ratio is 1.1) 38.5

The present invention is also an effective means of improving thequality of manufactured optical fibers by increasing the durability andthe useful life of such fibers and preforms, particularly under hightemperature conditions. The present invention achieves this throughmicrowave plasma deposition of a coating of SiO_(x)N_(y) on the surfaceof a preform. The plasma created in the present invention contains “hot”electrons that possess high kinetic energy. These electrons are capableof stimulating effective N₂ dissociation in the gas phase by electronimpact. This is beneficial in that it leads to highly effectiveintroduction of atomic nitrogen in a glass matrix. This reliablechemosorption of nitrogen is carried out in silica layers grown at lowreactor temperature (about 1200 C).

In the disclosed method, a SiO_(x)N_(y) layer is deposited on theflourine-doped silica cladding layer previously deposited on the silicarod. This SiO_(x)N_(y) deposition is accomplished using the samemicrowave plasma-chemical deposition method, at pure conditions, as ispreviously described in the present invention's method for claddingdeposition. The preferred thickness of a protective coating for a 30 mmdiameter preform is more than 0.3 mm. The present invention is moredesirable than other prior art methods for deposition of a protectiveSiO_(x)N_(y) coating in that its characteristics include a highdeposition rate and a greater uniformity of the deposited protectivecoating. The present invention can be accomplished in one step, is avery clean process, and produces a high quality coating.

After deposition of a SiO_(x)N_(y) coating, 30–40 mm diameter preformscan be drawn into fibers with great speed, at drawing rates of greaterthan 100 m/min, because all protective coatings are initially on thepreform. As a result, the present invention improves the productivity oflightguide manufacture, improves optical fiber quality, and reduces thebase cost of fibers.

Optical fiber preform manufacturing according to the present inventionis carried out by the following method. The parameters used in thefollowing description are for illustration purposes, and do not limitthe invention to those materials or parameters.

Reactor 102 and silica preform rod 101 are mounted coaxially in electricfurnace 108 (not shown). The SiO₂—F/SiO₂ preform is produced with themethod described earlier using low pressure microwave plasma deposition.In this example, the total diameter of the produced preform, includingthe cladding, is 36 mm. After the cladding has been deposited, a gasmixture such as N₂+O₂+SiCl₄ or N₂+air+SiCl₄ is supplied to the microwaveplasma zone 107 for deposition of a protective SiO_(x)N_(y) layer. Thefurnace temperature is set at approximately 1200° C. In principle, theSiO_(x)N_(y) deposition process is analogous to the SiO₂—F depositionprocess described above. However, the present method is distinguishablein that the microwave power and the furnace temperature duringSiO_(x)N_(y) deposition are increased while the reactor (silica tube)temperature is held constant at 1200° C. In this process, N₂ is thebasic carrying gas and O₂ or air are the dopants. It is necessary,during the deposition process, to decrease the O₂ content of the processgas by up to 7–10% because full SiCl₄ oxidation to SiO₂ occurs at higherO₂ concentrations. In cases where there is a higher O₂ concentration, N₂does not take part in the chemical reaction. Also, the nitrogenconcentration in the silica glass should be increased gradually to avoidcracking of the deposited SiO_(x)N_(y) layers during cooling. This is adanger because SiO_(x)N_(y) has a much higher thermal expansioncoefficient than does pure silica.

The produced preform should also be gradually and uniformly cooled afterthe deposition process is complete. Examples of technological parametersof SiO_(x)N_(y) layer deposition on SiO₂—F/SiO₂ preform surfaces follow.

EXAMPLE 3

Nitrogen flow, cm³/min 700 N₂-flow in SiCl₄-bubbler, cm³/min 300 Airflow, cm³/min 540 Reactor temperature, C. 1200 Preform diameter, mm 36Reactor diameter, mm 50 Length of deposition zone, mm 1200 Frequency ofgenerator, MHz 2450 Power of generator, kW 6.0 Speed of movement ofplasma generator, m/min 2.0 Pressure, torr 6.0 Thickness of Si—O—Ncoating, mm 0.36

Investigations have shown that the nitrogen concentration in theSiO_(x)N_(y) glass matrix is typically 8–10%. This nitrogen presenceprevents hydrogen diffusion through the optical fiber because atomicnitrogen introduced in the silica glass matrix fills all the gaps of theglass structure and thus prevents hydrogen from diffusing through thisprotective layer. As a result, optical losses can be greatly reduced oreliminated. For example, fibers with a SiO_(x)N_(y) protective coatingexhibited no increases in optical losses when used in an H₂ atmosphereand at temperatures of 200° C.

SiO_(x)N_(y) layers can be deposited directly on the outside surface ofa silica tube prior to cladding deposition using the method describedabove. In this case a bare silica rod should be introduced into thereactor tube to preclude any plasma formation within the tube. To avoidmicrowave discharge in the small gap between the rod and the tube it isnecessary to sustain atmospheric pressure in the gap. Other than theprescribed pressure, the remaining parameters listed in Example 3 can beused for this deposition process.

SiO_(x)N_(y) glass can be also deposited on capillaries (note that, inorder to prevent a plasma from forming within the capillary, atmosphericpressure inside the capillary should be maintained during deposition),silica rods and optical fibers (fiber diameter is preferably 2–8 mm),and planar quartz substrates (100×10×1 mm and other sizes) by using thescanning E₀₁₀ resonator. These articles must be coaxially located withinthe silica tube along the tube axis under microwave plasma transference(pressure is 1–100 torr). In a preferred embodiment, the tube diametermay be 20×17 mm, 23×20 mm. The following example lists possible processparameters for protective layer deposition on a capillary.

EXAMPLE 4

The silica tube diameter, mm 20 × 17 The silica capillary diameter, mm6.0 Nitrogen flow, cm³/min 60 Nitrogen flow in SiCl₄-bubbler, cm³/min 30Air flow, cm³/min 60 The silica tube temperature, C. 1200 Oxygen contentin gas mixture, % 8

The deposition rate of the SiO_(x)N_(y) glass layer in this example is0.5 microns/min, and the thickness of the coating deposited may be from0.1–1 micron. The tube reactor is not needed in this process. Input andoutput hole diameters of the E₀₁₀ resonator must be decreased inaccordance with any reduction in the silica tube diameter. In this casemetal screens are inserted into resonator holes. The metal screens inthe resonator should be coated by a microwave absorbing material such asgraphite. A gas mixture of SiH₄+NO+NH₃ is a preferred mixture fordeposition of a SiO_(x)N_(y) layer with high nitrogen content. Thedeposition temperature should be between 800–900° C. The index ofrefraction and the percentage of nitrogen in the SiO_(x)N_(y) layer arelinearly dependent on the ratio of NH₃ to NO (NH₃/NO) used in theprocess gas mixture. For example, at ratio NH₃/NO=1000, the refractiveindex “n” of the SiO_(x)N_(y) layer is 1.85 (the wavelength is 546 nm).The layers deposited with these parameters were 50% atomic nitrogen. AnySiO_(x)N_(y) compositions can be manufactured with the present method.These compositions can be appreciated with sufficient accuracy byn-value.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to the precise embodiments, and that various changes andmodifications may be effected therein by those skilled in the artwithout departing from the scope or spirit of the invention as definedin the appended claims.

1. A method of manufacturing optical fiber preforms comprising the stepsof: a. arranging a rod coaxially within a hollow cylindrical reactor,creating a cavity between said rod and said reactor; b. reducingpressure in said cavity to a predetermined level; c. enclosing said rodand said reactor coaxially within a hollow cylindrical furnace, andheating said rod and said reactor to a specified temperature; d.introducing a process gas mixture in said cavity, wherein said mixtureis comprised of materials to be deposited; e. generating a plasma insaid cavity by simultaneously exciting an E₀₁₀ electric field mode waveand an E₀₁ electric field type plasma surface wave in said cavity bymeans of a microwave system, wherein said plasma surface wave is locatedat an interface between said rod and said plasma; f. wherein an electricfield is at a maximum intensity at a radial distance from the centralaxis of said cavity equal to a distance where said rod-plasma interfaceis located; and wherein said intensity exponentially diminishes as theradius increase to said reactor and as the radius decreases toward saidaxis; and g. moving said plasma along said cavity in a directionparallel to the axis of said cavity, thereby uniformly depositing saidmaterials on said starting body.
 2. A method of manufacturing opticalfiber preforms according to claim 1, wherein said microwave systemcomprises a resonator, a waveguide, and a microwave source.
 3. A methodof manufacturing optical fiber preforms according to claim 1, whereinsaid starting body is a solid silica rod.
 4. A method of manufacturingoptical fiber preforms according to claim 1, wherein said E₀₁₀ mode andplasma surface wave E₀₁ are created simultaneously in said microwavesource by symmetrical and homogeneous excitation from said microwavesource through a system of coupled inputs of energy.
 5. A method ofmanufacturing optical fiber preforms according to claim 1, wherein saidmaterial is deposited on said rod to form a reflective cladding.
 6. Amethod of manufacturing optical fiber preforms according to claim 5,wherein said material is fluorine doped silica.
 7. A method ofmanufacturing optical fiber preforms according to claim 1, wherein saidmaterial is deposited on said rod to form a protective layer.
 8. Amethod of manufacturing optical fiber preforms according to claim 7,wherein said protective layer is deposited by the method in claim 1after a reflective cladding is deposited.
 9. A method of manufacturingoptical fiber preforms according to claim 7, wherein said process gasmixture consists of SiH₄, NH₃, and NO.
 10. A method of manufacturingoptical fiber performs according to claim 7 wherein said protectivelayer is made from a process gas mixture selected from the groupconsisting of (N₂+O₂+SiCl₄) and (N₂+air+SiCl₄).
 11. A method ofmanufacturing optical fiber preforms according to claim 10 wherein saidprotective layer is a film of SiO_(x)N_(y).